Based on the organism's capsular polysaccharide, twelve serogroups of N. meningitidis have been identified (A, B, C, H, I, K, L, 29E, W135, X, Y and Z). Group A is the pathogen most often implicated in epidemic disease in sub-Saharan Africa. Serogroups B and C are responsible for the vast majority of cases in USA and in most developed countries. Serogroups W135 and Y are responsible for the remaining cases in USA and developed countries.
A tetravalent vaccine of capsular polysaccharides from serogroups A, C, Y and W135 has been known for many years [1,2]. Although effective in adolescents and adults, it induces a poor immune response and short duration of protection and cannot be used in infants [e.g. ref. 3] because polysaccharides are T cell-independent antigens that induce a weak immune response which cannot be boosted. The polysaccharides in this vaccine are not conjugated [4].
Conjugate vaccines against serogroup C have been approved for human use, and include Menjugate™ [5], Meningitec™ and NeisVac-C™. Mixtures of conjugates from serogroups A+C are known [6-8] and mixtures of conjugates from serogroups A+C+W135+Y have been reported [9-13].
While meningococcal conjugates are well known, they have not yet been fitted into existing pediatric immunisation schedules, which for developed countries typically involve: hepatitis B vaccine at birth; and, starting at 2 months, all of diphtheria/tetanus/pertussis (D-T-P), H. influenzae type, b (Hib) conjugate, inactivated poliovirus and pneumococcus conjugates at 2 months.
When adding conjugated vaccines to existing immunisation schedules, however, the issue of carrier-induced epitopic suppression (or “carrier suppression”, as it is generally known) must be addressed, particularly suppression arising from carrier priming. “Carrier suppression” is the phenomenon whereby pre-immunisation of an animal with a carrier protein prevents it from later eliciting an immune response against a new antigenic epitope that is presented on that carrier [14].
As reported in reference 15, where several vaccine antigens contain the same protein component (being used as an immunogen and/or as a carrier protein in a conjugate) then there is the potential for interference between those antigens. In reference 15, the immune response against an antigen that was conjugated to a tetanus toxoid (Tt) carrier was suppressed by pre-existing immunity against Tt.
Reference 16 reports how a combination of D-T-P vaccines with a Hib conjugate vaccine was adversely affected where the carrier for the Hib conjugate was the same as the tetanus antigen from the D-T-P vaccine. The authors concludes that this “carrier suppression” phenomenon, arising from interference by a common protein carrier, should be taken into account when introducing vaccines that include multiple conjugates.
In contrast to references 15 and 16, reference 17 reported that priming with tetanus toxoid had no negative impact on the immune response against a subsequently-administered Hib-Tt conjugate, but suppression was seen in patients with maternally acquired anti-Tt antibodies. In reference. 18, however, an “epitopic suppression” effect was reported for a Tt-based peptide conjugate in patients having existing anti-Tt antibodies resulting from tetanus vaccination.
In reference 19, it was suggested that a conjugate having CRM197 (a detoxified mutant of diphtheria toxin) as the carrier may be ineffective in children that had not previously received diphtheria toxin as part of a vaccine (e.g. as part of a D-T-P or D-T vaccine). This work was further developed in reference 20, where a carrier priming effect by D-T immunisation was seen to persist for subsequent immunisation with Hib conjugates.
In reference 21, the authors found that pre-immunisation with a diphtheria or tetanus toxoid carrier protein reduced the increase in anti-Hib antibody levels after a subsequent immunisation with the Hib capsular saccharide conjugated to those carriers, with IgG1 and IgG2 being equally affected. Responses to the carrier portions of the conjugates were also suppressed. Furthermore, a more general non-epitope-specific suppression was seen, as pre-immunisation with one conjugate was seen to affect immune responses against both the carrier and saccharide portions of a second conjugate that was administered four weeks later.
The use of different carrier proteins in a single multivalent pneumococcal conjugate vaccine is reported in reference 22, with multiple carriers being used in order to avoid carrier suppression. The authors predict that there is a maximum load of a carrier protein that can be tolerated in a multivalent conjugate vaccine without giving rise to negative interference. In reference 23 it was reported that pneumococcal conjugate vaccines including mixed carrier proteins elicited, in parallel to the anti-pneumococcus response, unintentional booster responses to the carriers.
In reference 24, an investigation of whether diphtheria and tetanus boosters could be administered with monovalent meningococcal serogroup C conjugates, it was found that titres against the meningococcal conjugate were reduced where the carrier was tetanus toxoid carrier and the patient had received prior immunisation with a tetanus-containing vaccine.
Finally, reference 25 reports that “prior exposure to the carrier protein can either enhance or suppress antibody response to polysaccharides administered in saccharide-protein conjugates”. The conjugates used in reference 25 used tetanus toxoid or the CRM197 mutant as carrier protein.
The situation concerning carrier priming and/or suppression is thus confused, and it remains unclear whether any particular conjugate will suffer from carrier suppression or will benefit from a carrier priming enhancement. Meningococcal conjugate vaccines will not be in a position to be integrated into or added to existing pediatric immunisation schedules until this issue is addressed. Furthermore, as meningococcal conjugates are to be administered as tetravalent mixtures (i.e. four different conjugates) then the potential for carrier suppression becomes even more of a risk.
In addition to the problem of priming with a carrier having a negative impact on immune responses against saccharide conjugates, the reverse can also occur i.e. immunisation with a conjugate can have a negative impact on immune responses against the carrier [26].